The present invention relates to gas turbines and, more particularly, to apparatus for cooling a sealing frame of a gas turbine combustor transition piece.
A gas turbine conventionally includes a compressor to compress ambient air to relatively high pressure. The compressed air is mixed with fuel in a combustor where the fuel is burned to produce a high-temperature energetic flow of uncombusted air and products of combustion. This flow is employed to drive a turbine which, in turn, provides power to drive the compressor and may also turn an output shaft or be directed through a nozzle to produce thrust.
In industrial gas turbines, the combustion of fuel takes place in a plurality of burner cans tangentially spaced about the apparatus. The hot gases from the conventionally cylindrical burner cans are directed to the turbine stage through transition pieces which, in concert, convert the plurality of cylindrical flow fields from the burner cans into a substantially uniform 360 degree annular flow field entering the turbine stage. The burner cans and the transition pieces are disposed in a plenum containing the compressed air from the compressor.
The downstream ends of the transition pieces are shaped so that, when assembled into a 360 degree assembly, their outer and inner perimeters form concentric circles. The transverse edges of adjacent transition pieces abut to form radii of the concentric circles. In order to prevent leakage of compressed air past the downstream ends of the assembled transition pieces, outward-directed flanges are provided at the downstream ends of the transition pieces each forming a frame. Seals engage the flanges at the outer and inner perimeters of the concentric circles as well as between adjacent transition pieces.
One of the problems limiting the working life of transition pieces is stress creep deflection of the transition piece frame. Such stress creep deflection is believed to be caused by the elevated temperature in the vicinity of the sealing portions of the transition piece frame. With time in use, the distortion of the transition piece frame becomes so great that excessive leakage flow of compressed air from the plenum passes the seals and enters the flow of hot gases entering the turbine stage. This leakage flow is capable of distorting the temperature profile of the hot gases exiting the transition piece in an unpredictable way, making it cooler in some regions of the flow field and hotter in other regions. This distortion of the temperature profile is capable of compromising the life of downstream parts.